Glycyl tRNA synthetase regulation. A study of glycycl tRNA synthetase has revealed that two forms of this enzyme are interconverted in yeast cells as part of the operation of a regulatory mechanism that should be vital in governing the synthesis of proteins. The cyclic interconversion is now being studied in vitro. Evolution. Evidence has been obtained that a number of 5'-substituted uracils have a substantial affinity and modest discriminating capacity for certain hydrophobic peptides. These results support the possibility of a direct template-peptide coding system such as we proposed earlier as a mechanism for the early evolution of life and origin of the genetic code.